1. Field of the Invention
The present invention relates to the configuration of cemented lens group, in particular, to the configuration a cemented lens group constituted by three lens elements.
2. Description of the Related Art
A cemented lens group formed by cementing three lens elements has been used in a high-performance lens system, e.g., a Sonar-type photographing lens system, an Abbe-type eyepiece optical system and so forth, since the cemented lens group shows excellent performance in correcting aberrations, and causes few ghost images due to a surface reflection of lens elements thereof.
General procedures of cementing lens elements are as follows:
(i) applying a drop of cement onto a surface to be cemented of one lens element;
(ii) placing another lens element onto the surface of the lens element in (i);
(iii) removing bubbles trapped inside the cement;
(iv) centering the lens elements; and
(v) hardening the cement by irradiating ultraviolet or by heating.
In the process of removing bubbles from the cement, one lens element is moved in radial directions so that bubbles together with the cement are forced to exit from the periphery of the cemented surfaces. Consequently, in such a cement-removing process, cement is inevitably overflown from the periphery of the cemented surfaces.
If the cement is hardened while a portion thereof remains overflown from the periphery of the cemented surfaces, the overflown portion will also be hardened, and will form burrs thereon. If this happens, the cemented lens group cannot be assembled into a lens supporting frame. Further, in the cemented lens group of the prior art, since cement may flow over a lens surface, the overflown cement has to be wiped off. For example, as shown in FIGS. 5 and 6, the conventional cemented lens groups, including three round lens elements L1(L4), L2(L5) and L3(L6), have two cemented surfaces from which cement may overflow. Therefore the process of wiping off the cement becomes complicated, which increases production costs of the cemented lens groups.
It is an object of the present invention to provide an inexpensive cemented lens group of three lens elements, which does not require a cement-removing process for overflown cement.
The present invention has been developed based on the conception that in a cemented lens group of three lens elements, if the diameters of the two lens elements other than the intermediate lens element, i.e., the front-most and rear-most lens elements, are arranged to be larger than the height, in the radial direction, of the overflown cement from the surfaces to be cemented, the overflown cement does not hamper the assembling process of the cemented lens group into a lens supporting frame.
In order to achieve the above object, there is provided a cemented lens group formed by cementing three round lens elements. In the cemented lens group, the diameter of the intermediate lens element is made smaller than those of the other two lens elements.
More specifically, the difference of the diameters between the intermediate and other two lens elements is determined so that the cement overflown from the surfaces to be cemented can be held in the groove formed by the intermediate lens element and other two lens elements. Further, it is practical to make the diameters of the other two lens elements the same.
The cemented lens group according to the present invention preferably satisfies the following condition:
0.01 less than |xcex94D/D| less than 0.3xe2x80x83xe2x80x83(1)
wherein
xcex94D designates the difference of the diameters of the intermediate lens element and the other two lens elements, or in the case where the other two lens elements have different diameters, the difference of the diameters of the intermediate lens element and one of the other two lens elements which is smaller than the rest thereof; and
D designates the diameter of the intermediate lens element.
The present disclosure relates to subject matter contained in Japanese Patent Application No.Hei-11-219649 (filed on Aug. 3, 1999) which is expressly incorporated herein by reference in its entirety.